Most of the good water detectors don't drop below a certain resell value for many years.....A new Excal II will cost $1200, a 4-6 year old Excal will still cost you $500-$700 used. If you look constantly every day several times a day you can find one for $400-$500 on craigslist, Ebay or another source because the seller doesn't know the resale value, but those go very quickly, and well not sit there for longer then a couple hours...
It is the same way with the Beach Hunter ID, you will really have to search hard to find one for $400.....
One of the advantages to buying a high quality water detector is the resale value, you can buy them, use them for couple years and get most if not all your money out when you sell.........Personally, and it is just my personal opinion I would stay away from the cheaper water detectors as they will have almost no resale value...(Any thing that has a face looking back at you on the controls.....)